1. Field of the Invention
This invention relates to diaphragm pumps, and more particularly to intermittently utilized continuous running diaphragm pumps of the type used in connection with spraying guns where the gun demand is intermittent while the drive force to the pump remains engaged.
2. Prior Art
Diaphragm spray pumps, particularly paint spray pumps, are known to the art and include devices which regulate the pumping system during standby state when the spray gun is closed but the driving motor is running. This invention relates to such pumps and to a method and device for regulating the output of diaphragm pumps used for delivering working substances, particularly liquids for airless spraying by means of high pressure spray guns. Such pumps generally comprise two chambers separated by a movable diaphragm. A first of the chambers is filled with a drive fluid which is alternately loaded and unloaded (pressurized and unpressurized) by an oscillating piston. A second chamber is designated as a driven fluid chamber or working substance chamber. Additionally, the pumps include a pressure limiting valve which discharges drive fluid from the drive chamber to a reservoir when the pressure in the drive chamber exceeds the setting of the limit valve. A closable intake aperture or restricted intake aperture is provided for supply of drive fluid from the reservoir to the drive chamber.
Such diaphragm pumps are distinguished from other pumps in that the driven fluid does not come into contact with the oscillating pump piston. This is particularly advantageous when the driven fluid, as often is the case, has a certain corrosiveness or abrasiveness. Difficulties, however, can arise from such uses when the delivery of the pumped fluid is frequently interrupted while the pump drive remains in operation during the interrupted (standby) state. A distinct example of such usage is the spraying of paints and lacquers by means of airless high pressure guns. During the painting operation, the gun is frequently opened and closed whereas, in contrast, the pump motor remains in constant operation. If the gun is closed, i.e. liquid is no longer being sprayed, then the pressure in the pumped fluid chamber increases to the point that the diaphragm can no longer arc into the pumped fluid chamber. The diaphragm is therefore brought to a standstill. In so doing, however, there exists the necessity of opening the pressure limit valve of the drive fluid chamber so that an excess amount of drive fluid corresponding to the displacement volume of the piston can be discharged from the drive chamber through the limiting valve to the reservoir. At the next successive suction stroke of the piston, absent other controls, that same amount of drive fluid will again be sucked into the intake aperture of the drive fluid chamber. In this type of construction, there thus ensues a continuous standby state circulation of driving fluid from the driving chamber through the pressure limit valve to the reservoir and thence from the reservoir through the intake aperture back to the drive chamber. The energy generated by the pump drive in such a standby state will be converted into a fluid circulation which in turn converts the energy to heat upon passing through the pressure limit valve. The end result is that the drive energy requirement is high during standby and a continuously high heat input to the drive fluid will occur.
In order to avoid excessive heating of the drive fluid during such standby operation, it has been known to provide special cooling apparatus. In such systems, a reduction of energy input during standby is not to be achieved. Another known method for avoiding overheating is through the utilization of a closable intake aperture from the reservoir into the driving fluid chamber. In such construction, a valve or an intake slot traversed by the piston can be used which has a smaller flow capacity than the pressure limit valve such that the amount of fluid discharged by the pressure limiting valve on the pressure stroke of the piston cannot be entirely replaced on a single suction stroke of the piston. As is known in the art (U.S. Pat. Nos. 3,254,845; 3,367,270) the reduction in full volume through the intake valve is such that an under pressure will arise in the driving fluid during the suction phase movement of the piston to the extent that a change in the nature of the drive fluid is said to occur. Independently the question of the nature of the change, one can still proceed from the fact that the circulation during the standby phase is in fact lower dependent upon how strongly the intake aperture is choked. At any rate, what is achieved with this choking method is that the driving fluid will be less heated during the standby phase and that the output power of the pump motor or drive will be reduced during standby.
Although the above method has advantages, it has a significant disadvantage. When the gun is reopened after standby operation, a considerable time period is required until the full amount of the drive fluid can be reintroduced through the reduced intake aperture to the drive chamber. The result of this is a pressure drop in the driven fluid chamber. This pressure incidence in the driven fluid chamber is increased the more strongly the intake aperture is choked. Thus, one will always have to strike a compromise between the degree of intake aperture restriction and the length and extent of pressure change upon reversion to a spraying status from a standby status.
Another previously disclosed method includes therein the mixing of a certain percentage of air into the drive fluid (U.S. Pat. Nos. 3,680,981; RE 29,055). The addition of air makes the driving fluid somewhat elastic. As a result of the compressibility of the air, it is not necessary in a standby phase to discharge from the drive fluid at every pressure stroke an amount which corresponds to the entire displacement volume of the piston so that the fluid circulation, and thus the heating and power output are reduced. The pressure change upon reopening of the gun is reduced or avoided by this method, however, here also, a compromise must be made where, given a small amount admixed air, the fluid circulation in the standby state is still considerable whereas, given too great an amount of admixed air, too great a power reduction will occur during the actual working phase of the pump. Experience has indicated that the air admixture method, in particular, or combination of the air admixture and driving fluid change methods gives satisfactory results when used in connection with diaphragm pumps of low or moderate output but that difficulties occur when diaphragm pumps of higher output are used. Moreover, particularly using high output diaphragm pumps, there is an added that changes from small to large spray nozzles have an effect which is analogous to the extreme case of the change from a closed to an open gun.
It would therefore be a distinct advance in the art of intermittent demand continuous drive diaphragm pumps to provide a device and method of operation which reduces or eliminates many of the difficulties heretofor encountered.